The present invention relates to the electrical, electronic and computer arts, and more specifically, to magnetic random-access memory (MRAM).
In magnetic random-access memory, the magnetic tunnel junction (MTJ) is the cornerstone of the bit cell array. The MTJ can have two different values of resistance: low resistance, called Rparallel (Rp), and high resistance, called Rantiparallel (Rap). Each resistance corresponds respectively to a logic state: ‘1’ or ‘0’. To read the state of the bit cell, two different techniques are conventionally used. The first technique is known as the self-reference readout technique (it does not utilize a reference cell). The self-reference readout technique stores the initial data of the bit cell, adds a small offset of resistance to the reading path, reads the bit cell again, and compares the two values. This first technique may have a better yield than other conventional techniques, but the speed performance can be very low.
The second technique utilizes a reference cell The reference cell can be defined as a resistance value (Rref) which is between Rp and Rap (Rp<Rref<Rap). Ideally, the value of Rref is centered in the middle of Rp and Rap (Rref=(Rp+Rap)/2) to establish a suitable read margin. In practice, Rref is not necessarily centered in the middle of Rp and Rap due to the process variations of Rp and Rap.
FIGS. 1A and 1B are schematic diagrams 100, 150 of conventional read circuits incorporating reference cells. Note that “TMR” refers to tunnel magnetoresistance. A resistance Rbc of a bit cell being read results in a current Ibc that is compared by readout circuit 104 to a reference current Iref. The reference current Iref is determined by the overall resistance Rref of the reference MTJ 108. FIG. 1C shows the ideal case where the reference current Iref is based on the resistance Rref and is centered between I(Rp) and I(Rap). FIG. 1D illustrates an example histogram of MRAM devices where the values of Rp, Rap, and Rref for each MRAM device vary due to process variations. The values of Rp, Rap, and Rref vary on the curve leading to a non-centered value of Rref, a decreased read margin and, consequently, an increased read error rate. Also, the reference cell is based on the Rp and Rap resistances which means that the reference cell requires a bidirectional writing circuit which is often designed to satisfy the MTJ writing conditions.